As phosphorescent platinum(II) complexes can be thermally stable and have high emission quantum efficiency, they are potential dopant materials for OLED application. However, due to square planar geometry, platinum(II) complexes have a high self-aggregation tendency, which results in a red-shift in emission λmax; excimer emission; and low device efficiency.
Considerable effort has been made to deal with this issue; bulky groups such as tert-butyl group(s) and non-planar phenyl group(s) have been added to the platinum(II) complexes. Nevertheless, most of them are not successful. In 2010, Che added tert-butyl group(s) in red-emitting platinum(II) material. [Chem. Eur. J. 2010, 16, 233-247] However, close intermolecular stacking π-π interactions were still observed in the X-Ray crystal structure which means the problem cannot be resolved. In the same year, Huo reported a class of platinum(II) materials containing a non-planar phenyl ring, however excimer emission appears in doping concentration more than 4 wt. % and severe triplet-triplet annihilation was observed even in a device with a mix host, which means this approach cannot resolve the problem [Inorg. Chem. 2010, 49, 5107-5119]. In 2013, Xie prepared new emitters containing two non-planar Spiro-structures. [Chem. Commun. 2012, 48, 3854-3856] However, the devices fabricated by this emitter show serious efficiency roll-off of greater than 50% which indicates adding non-planar group(s) may be able to reduce self-aggregation. In the same year, Che combined the two approaches and used a new, robust (O^N^C^N) ligand system to prepare new platinum(II) materials. In which, one of the emitters shows a wide doping window and slow efficiency roll-off [Chem. Commun. 2013, 49, 1497-1499]. However, the maximum current efficiency of the device can only achieve 66.7 cd/A, even the emission quantum efficiency of the device is 90%. If the self-aggregation effect is resolved, approximately, or even greater than 100 cd/A can be obtained with this emission quantum efficiency. In 2014, Che constructed a new ligand structure containing a Spiro linkage in the ligand which resolved the self-aggregation problem; a green device with power efficiency up to 126 lm/W has been fabricated. [Chem. Sci. 2014, 4819-4830] These examples show that no universal approach can be used in all platinum(II) complexes. A method that works in system A may cause bigger problems in system B.
Furthermore, the systems mentioned above are not suitable for developing blue emitting platinum(II) complexes. Those ligand systems are complicated and have long π-conjugation, such as tetradentate ligands. The emission λmax of the complexes developed by these approaches are larger than 500 nm and thus no blue emitter can be prepared.
For blue emitting platinum(II) complexes, Thompson reported a platinum complex which showed blue emission in dilute solution in 2002 [New J. Chem. 2002, 26, 1171-1178]. Due to strong excimer emission, instead of blue OLED, only single emitter white OLED can be fabricated. In 2009, Bhansali developed Pt(ptp)2 which showed blue emission in dilute solution, but because excimers appear at 2.5% dopant concentration, only yellow to orange OLED can be fabricated in a reasonable doping concentration (greater than or equal to 5% by weight) [Appl. Phys. Lett. 2009, 95, 233304]. In 2012, Li reported Pt-16 which was fabricated into blue OLED [Organ. Electron. 2012, 1430-1435]. However, it was later proved that, due to excimer emission, blue emission cannot be maintained using these approaches when the doping concentration was increased to more than 2% by weight, meaning that only white OLED can be obtained. [Adv. Mater. 2013, 25, 2573-2576].
Besides excimer emission, changing the chemical structure of the complexes also results in a red-shift in monomer emission which is not good for blue emitting material development. For example, changing the auxiliary ligand of (tridendate ligand)Pt (auxiliary ligand) type complexes from halide [Inorg. Chem. 1999, 38, 4046-4055] to —C≡C—R [J. Am. Chem. Soc. 2004, 126, 4958-4971] proved largely red-shift emission λmax (up to 65 nm red-shift was observed). Therefore, consideration of fixing the excimer emission issue in this way is avoided.